Showing papers on "Epitaxy published in 2009"

Silicon Nanowires: A Review on Aspects of their Growth and their Electrical Properties

Abstract: This paper summarizes some of the essential aspects of silicon-nanowire growth and of their electrical properties. In the first part, a brief description of the different growth techniques is given, though the general focus of this work is on chemical vapor deposition of silicon nanowires. The advantages and disadvantages of the different catalyst materials for silicon-wire growth are discussed at length. Thereafter, in the second part, three thermodynamic aspects of silicon-wire growth via the vapor–liquid–solid mechanism are presented and discussed. These are the expansion of the base of epitaxially grown Si wires, a stability criterion regarding the surface tension of the catalyst droplet, and the consequences of the Gibbs–Thomson effect for the silicon wire growth velocity. The third part is dedicated to the electrical properties of silicon nanowires. First, different silicon nanowire doping techniques are discussed. Attention is then focused on the diameter dependence of dopant ionization and the influence of interface trap states on the charge carrier density in silicon nanowires. It is concluded by a section on charge carrier mobility and mobility measurements.

Quintuple-layer epitaxy of thin films of topological insulator Bi2Se3

Abstract: Atomically smooth, single crystalline Bi2Se3 thin films were prepared on Si(111) by molecular beam epitaxy. Scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron emission spectroscopy, and Raman spectroscopy were used to characterize the stoichiometry and crystallinity of the film. The films grow in a self-organized quintuple layer by quintuple-layer mode, and atomically smooth films can be obtained, with controllable thickness down to one quintuple layer (∼1 nm).

Electronic doping and scattering by transition metals on graphene

Abstract: We investigate the effects of transition metals (TM) on the electronic doping and scattering in graphene using molecular-beam epitaxy combined with in situ transport measurements. The room-temperature deposition of TM onto graphene produces clusters that dope $n$ type for all TM investigated (Ti, Fe, and Pt). We also find that the scattering by TM clusters exhibits different behavior compared to $1/r$ Coulomb scattering. At high coverage, Pt films are able to produce doping that is either $n$ type or weakly $p$ type, which provides experimental evidence for a strong interfacial dipole favoring $n$-type doping as predicted theoretically.

Quintuple-layer epitaxy of high-quality Bi2Se3 thin films for topological insulator

Abstract: We report the growth of atomically smooth, single crystalline Bi2Se3 thin films on Si(111) by using molecular beam epitaxy. Scanning tunneling microscopy, low-energy electron diffraction, X-ray photoelectron emission spectroscopy and Raman spectroscopy were used to characterize the stoichiometry and crystallinity of the film. The film grows in a self-organized quintuple-layer by quintuple-layer mode, and atomically smooth film can be obtained with the thickness down to one quintuple-layer (~1nm).

Growth of Core–Shell InP Nanowires for Photovoltaic Application by Selective-Area Metal Organic Vapor Phase Epitaxy

Abstract: We report on the formation of core–shell pn junction InP nanowires using a catalyst-free selective-area metalorganic vapor-phase epitaxy (SA-MOVPE) method. A periodically aligned dense core–shell InP nanowire array was fabricated and used in photovoltaic device applications. The device exhibited open-circuit voltage (VOC), short-circuit current (ISC) and fill factor (FF) levels of 0.43 V, 13.72 mA/cm2 and 0.57, respectively, which indicated a solar power conversion efficiency of 3.37% under AM1.5G illumination. This study demonstrates that high quality core–shell structure nanowire fabrication is possible by SA-MOVPE and that the nanowire arrays can be used in integrated nanowire photovoltaic devices.

Growth of homoepitaxial SrTiO3 thin films by molecular-beam epitaxy

Abstract: We report the structural properties of homoepitaxial (100) SrTiO3 films grown by reactive molecular-beam epitaxy (MBE). The lattice spacing and x-ray diffraction (XRD) rocking curves of stoichiometric MBE-grown SrTiO3 films are indistinguishable from the underlying SrTiO3 substrates. Off-stoichiometry for both strontium-rich and strontium-poor compositions (i.e., Sr1+xTiO3+δ films with −0.2

Metalorganic Vapor Phase Epitaxy of III-Nitride Light-Emitting Diodes on Nanopatterned AGOG Sapphire Substrate by Abbreviated Growth Mode

Abstract: Metalorganic vapor phase epitaxial (MOVPE) growth of GaN on nanopatterned AGOG sapphire substrates was performed, and characteristics of the light-emitting diode (LED) devices grown on patterned sapphire and planar substrates were compared. The nanopatterned sapphire substrates were fabricated by a novel process (AGOG) whereby aluminum nanomesas were epitaxially converted into crystalline Al2O3 via a two-stage annealing process. The GaN template grown on the nanopatterned sapphire substrate was done via an abbreviated growth mode, where a 15-nm thick, low-temperature GaN buffer layer was used, without the use of an etch-back and recovery process during the epitaxy. InGaN quantum wells (QWs) LEDs were grown on the GaN template on the nanopatterned sapphire, employing the abbreviated growth mode. The optimized InGaN QW LEDs grown on the patterned AGOG sapphire substrate exhibited a 24% improvement in output power as compared to LEDs on GaN templates grown using the conventional method. The increase in output power of the LEDs is attributed to improved internal quantum efficiency of the LEDs.

Growth of high-quality SrTiO3 films using a hybrid molecular beam epitaxy approach

Abstract: A hybrid molecular beam epitaxy approach for atomic-layer controlled growth of high-quality SrTiO3 films with scalable growth rates was developed. The approach uses an effusion cell for Sr, a plasma source for oxygen, and a metal-organic source (titanium tetra isopropoxide) for Ti. SrTiO3 films were investigated as a function of cation flux ratio on (001) SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates. Growth conditions for stoichiometric insulating films were identified. Persistent (>180 oscillations) reflection high-energy electron diffraction oscillation characteristic of layer-by-layer growth were observed. The full widths at half maximum of x-ray diffraction rocking curves were similar to those of the substrates, i.e., 34 arc sec on LSAT. The film surfaces were nearly ideal with root mean square surface roughness values of less than 0.1 nm. The relationship between surface reconstructions, growth modes, and stoichiometry is discussed.

Weak epitaxy growth of organic semiconductor thin films

Abstract: The fabrication of organic semiconductor thin films is extremely important in organic electronic devices. This tutorial review—which should particularly appeal to chemists and physicists interested in organic thin-film growth, organic electronic devices and organic semiconductor materials—summarizes the method of weak epitaxy growth (WEG) and its application in the fabrication of high quality organic semiconductor thin films. WEG achieves the thin-film fabrication of disk-like organic semiconductor molecules with highly structural order, molecular level smoothness and large size domains on amorphous substrate. The organic field-effect transistor devices based on these thin films exhibit a high charge mobility that is comparable with their corresponding single-crystal devices. Moreover, it provides a way to produce organic superlattices.

Control and elimination of nucleation-related defects in GaP/Si(001) heteroepitaxy

Abstract: GaP films were grown on offcut Si(001) substrates using migration enhanced epitaxy nucleation followed by molecular beam epitaxy, with the intent of controlling and eliminating the formation of heterovalent (III-V/IV) nucleation-related defects—antiphase domains, stacking faults, and microtwins. Analysis of these films via reflection high-energy electron diffraction, atomic force microscopy, and both cross-sectional and plan-view transmission electron microscopies indicate high-quality GaP layers on Si that portend a virtual GaP substrate technology, in which the aforementioned extended defects are simultaneously eliminated. The only prevalent remaining defects are the expected misfit dislocations due to the GaP–Si lattice mismatch.

Light-emitting device

Abstract: PROBLEM TO BE SOLVED: To provide a light-emitting device employing silicon carbide, in particular, a light-emitting device such as a silicon carbide light-emitting element for the visible light wavelength range. SOLUTION: The light-emitting device is provided in which, for example, an n-type region and a p-type region formed by an ion implantation method constitute a pn junction on an insulating layer buried type semiconductor silicon carbide substrate. The semiconductor silicon carbide substrate is preferably a semiconductor silicon carbide substrate in which surface silicon of an SOI substrate is made thin, the extremely thin silicon layer is denatured into a silicon carbide layer through carbonization, and a silicon carbide epitaxial film is deposited thereon by a CVD (chemical vapor-deposition) method. COPYRIGHT: (C)2009,JPO&INPIT

A growth diagram for plasma-assisted molecular beam epitaxy of GaN nanocolumns on Si(111)

Abstract: The morphology of GaN samples grown by plasma-assisted molecular beam epitaxy on Si(111) was systematically studied as a function of impinging Ga/N flux ratio and growth temperature (730–850 °C). Two different growth regimes were identified: compact and nanocolumnar. A growth diagram was established as a function of growth parameters, exhibiting the transition between growth regimes, and showing under which growth conditions GaN cannot be grown due to thermal decomposition and Ga desorption. Present results indicate that adatoms diffusion length and the actual Ga/N ratio on the growing surface are key factors to achieve nanocolumnar growth.

Controllable p-type doping of GaAs nanowires during vapor-liquid-solid growth

Abstract: We report on controlled p-type doping of GaAs nanowires grown by metal-organic vapor-phase epitaxy on (111)B GaAs substrates using the vapor-liquid-solid growth mode. p-type doping of GaAs nanowires was realized by an additional diethyl zinc flow during the growth. Compared to nominally undoped structures, the current increases by more than six orders of magnitude. The transfer characteristics of fabricated nanowire metal-insulator-semiconductor field-effect transistor devices proved p-type conductivity. By adjusting the II/III ratio, controlled doping concentrations from 4.6×1018 up to 2.3×1019 cm−3 could be achieved at a growth temperature of 400 °C. The doping concentrations were estimated from electrical conductivity measurements applied to single nanowires with different diameters. This estimation is based on a mobility versus carrier concentration model with surface depletion included.

Thickness Estimation of Epitaxial Graphene on SiC Using Attenuation of Substrate Raman Intensity

Abstract: A simple, noninvasive method using Raman spectroscopy for the estimation of the thickness of graphene layers grown epitaxially on silicon carbide (SiC) is presented, enabling simultaneous determination of thickness, grain size, and disorder using the spectra. The attenuation of the substrate Raman signal due to the graphene overlayer is found to be dependent on the graphene film thickness deduced from x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) of the surfaces. We explain this dependence using an absorbing overlayer model. This method can be used for mapping graphene thickness over a region and is capable of estimating thickness of multilayer graphene films beyond that possible by XPS and Auger electron spectroscopy (AES).

Ferroelectric properties of epitaxial Pb(Zr,Ti)O3 thin films on silicon by control of crystal orientation

Abstract: Crystalline Pb(Zr,Ti)O3 (PZT) thin films between metallic-oxide SrRuO3 (SRO) electrodes were prepared using pulsed laser deposition on CeO2/yttria-stabilized zirconia buffered silicon (001) substrates. Different deposition conditions for the initial layers of the bottom SRO electrode result in an orientation switch. Either (110)- or (001)-oriented SRO thin films are obtained and the PZT films deposited on the bottom electrode continued both growth directions. The ferroelectric characteristics of the SRO/PZT/SRO capacitors are found to be strongly dependent on their crystalline orientation: PZT (001)-oriented thin films showed stable, high quality ferroelectric response, while the remnant polarization of the PZT (110)-oriented thin films only show high response after multiple switching cycles.

Nucleation mechanism of GaN nanowires grown on (111) Si by molecular beam epitaxy

Abstract: We have performed a real-time in situ x-ray scattering study of the nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy on AlN(0001)/Si(111). The intensity variation of the GaN diffraction peak as a function of time was found to exhibit three different regimes: (i) the deposition of a wetting layer, which is followed by (ii) a supralinear regime assigned to nucleation of almost fully relaxed GaN nanowires, eventually leading to (iii) a steady-state growth regime. Based on scanning electron microscopy and electron microscopy analysis, it is proposed that the granular character of the thin AlN buffer layer may account for the easy plastic relaxation of GaN, establishing that three-dimensional islanding and plastic strain relaxation of GaN are two necessary conditions for nanowire growth.

Transfer of graphene layers grown on SiC wafers to other substrates and their integration into field effect transistors

Abstract: This letter presents a simple method for transferring epitaxial sheets of graphene on silicon carbide to other substrates. The graphene was grown on the (0001) face of 6H-SiC by thermal annealing at 1550 °C in a hydrogen atmosphere. Transfer was accomplished using a peeling process with a bilayer film of gold/polyimide, to yield graphene with square millimeters of coverage on the target substrate. Raman spectroscopy provided evidence that the transferred material is single layer. Back gated field-effect transistors fabricated on oxidized silicon substrates with Cr/Au as source-drain electrodes exhibited ambipolar characteristics with hole mobilities of ∼100 cm2/V-s, and negligible influence of resistance at the contacts.

Triple-twin domains in Mg doped GaN wurtzite nanowires: structural and electronic properties of this zinc-blende-like stacking

Abstract: We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 34 eV for wurtzite to 32 eV in the twinned lattice regions These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering

Optical properties and structural characteristics of ZnMgO grown by plasma assisted molecular beam epitaxy

Abstract: Wurtzite Zn1−xMgxO thin films with Mg contents between x=0 and x=0.37 were grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy using a MgO/ZnMgO buffer layer. The a-lattice parameter is independent from the Mg concentration, whereas the c-lattice parameter decreases from 5.20 A for x=0 to 5.17 A for x=0.37, indicating pseudomorphic growth. The near band edge photoluminescence shows a blueshift with increasing Mg concentration to an emission energy of 4.11 eV for x=0.37. Simultaneously, the energetic position of the deep defect luminescence shows a linear shift from 2.2 to 2.8 eV. Low temperature transmission measurements reveal strong excitonic features for the investigated composition range and alloy broadening effects for higher Mg contents. The Stokes shift as well as the Urbach energy is increased to values of up to 125 and 54 meV for x=0.37, respectively, indicating exciton localization due to alloy fluctuations.

Specular electron scattering at single-crystal Cu"001… surfaces

Abstract: Epitaxial copper layers, 20 nm to 1.5-μm-thick, were grown on MgO(001) by ultrahigh vacuum magnetron sputter deposition at 80 °C. In situ electrical resistivity measurements indicate partial specular scattering at the Cu vacuum interface with a Fuchs–Sondheimer scattering parameter p=0.6±0.1. In situ deposition of 0.3 to 7.0-nm-thick Ta cap layers on the Cu surfaces leads to a resistivity increase, which is independent of the Ta thickness and is associated with a transition to completely diffuse surface scattering with p=0.0±0.1. The diffuse scattering is attributed to a “rough” electron potential at the Cu–Ta interface as well as to scattering into localized interface and surface states.

Giant Ge-on-Insulator Formation by Si–Ge Mixing-Triggered Liquid-Phase Epitaxy

Abstract: Formation of giant single-crystalline Ge-on-insulator (GOI) with 400 µm length is demonstrated by using seeding lateral liquid-phase epitaxy (L-LPE). High quality (100), (110), and (111) oriented single-crystals are obtained on Si substrates covered with SiO2. A mechanism based on the solidification temperature gradient due to Si–Ge mixing and the thermal gradient due to latent heat at the growth front is proposed. An additional experiment on quartz substrates well supports the importance of Si–Ge mixing as a trigger for giant L-LPE of Ge. This process opens up the possibility of Ge-based fully-depleted field-effect transistors with high electron and hole mobilities.

Molecular beam epitaxy of SrTiO3 on Si (001): Early stages of the growth and strain relaxation

Abstract: The molecular beam epitaxy of SrTiO3 (STO) layers on Si (001) is studied, focusing on the early stages of the growth and on the strain relaxation process. Evidence is given that even for optimized growth conditions, STO grows initially amorphous on silicon and recrystallizes, leading to the formation of an atomically abrupt heterointerface with silicon. Just after recrystallization, STO is partially strained. Further increase in its thickness leads to the onset of a progressive plastic relaxation mechanism. STO recovers its bulk lattice parameter for thicknesses of the order of 30 ML.

Metal-semiconductor transition in epitaxial ZnO thin films

Abstract: We report on the formation and transport properties of ZnO thin films which are grown by pulsed-electron beam deposition under a low residual oxygen pressure (10(-5) mbar). ZnO films presenting metallic conductivity at room temperature, and a metal-semiconductor transition at low temperature, were epitaxially grown on Al(2)O(3) single crystal substrates for growth temperatures in the 300-450 degrees C range. These results have been interpreted through the quantum corrections to conductivity in a disordered oxide conductor, implying first a high density of carriers leading to degenerate semiconductor ZnO films, and then a sufficient disorder in these films. These characteristics could be related to the nature of the ZnO films formed by pulsed-electron deposition: a high density of carriers related to an oxygen deficiency in the films and a high density of defects related to the high deposition rate of the pulsed-electron beam deposition method. (C) 2009 American Institute of Physics. [doi:10.1063/1.3259412]

Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate

Abstract: High crystalline a-plane (112¯0) GaN epitaxial layers with smooth surface morphology were grown on r-plane (11¯02) sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of x-ray rocking curve was measured as 407 arc sec along c-axis direction, and the root mean square roughness was 1.23 nm. Nonpolar a-plane InGaN/GaN light emitting diodes were subsequently grown on a-plane GaN template, and the optical output power of 0.72 mW was obtained at drive current of 20 mA (3.36 V) and 2.84 mW at 100 mA (4.62 V) with the peak emission wavelength of 477 nm.

Method of manufacturing a light emitting diode element

Abstract: A method of manufacturing a light emitting diode element is provided. A first patterned semi-conductor layer, a patterned light emitting layer, and a second patterned semi-conductor layer are sequentially formed on an epitaxy substrate so as to form a plurality of epitaxy structures, wherein the first patterned semi-conductor layer has a thinner portion in a non-epitaxy area outside the epitaxy structures. A passivation layer covering the epitaxy structures and the thinner portion is formed. The passivation layer covering on the thinner portion is partially removed to form a patterned passivation layer. A patterned reflector is formed directly on each of the epitaxy structures. The epitaxy structures are bonded to a carrier substrate. A lift-off process is performed to separate the epitaxy structures from the epitaxy substrate. An electrode is formed on each of the epitaxy structures far from the patterned reflector.